The present invention relates to a radio communication system and further relates to primary and secondary stations for use in such a system and to a method of operating such a system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that such techniques are equally applicable to use in other mobile radio systems.
There are two basic types of communication required between a Base Station (BS) and a Mobile Station (MS) in a radio communication system. The first is user traffic, for example speech or packet data. The second is control information, required to set and monitor various parameters of the transmission channel to enable the BS and MS to exchange the required user traffic.
In many communication systems one of the functions of the control information is to enable power control. Power control of signals transmitted to the BS from a MS is required so that the BS receives signals from different MS at approximately the same power level, while minimising the transmission power required by each MS. Power control of signals transmitted by the BS to a MS is required so that the MS receives signals from the BS with a low error rate while minimising transmission power, to reduce interference with other cells and radio systems. In a two-way radio communication system power control is normally operated in a closed loop manner, whereby the MS determines the required changes in the power of transmissions from the BS and signals these changes to the BS, and vice versa.
An example of a combined time and frequency division multiple access system employing power control is the Global System for Mobile communication (GSM), where the transmission power of both BS and MS transmitters is controlled in steps of 2 dB. Similarly, implementation of power control in a system employing spread spectrum Code Division Multiple Access (CDMA) techniques is disclosed in U.S. Pat. No. 5,056,109.
A problem with these known techniques is that at the start of a transmission, or after the transmission is interrupted, the power control loops may take some time to converge satisfactorily. Until such convergence is achieved data transmissions are likely to be received in a corrupted state if their power level is too low, or to generate extra interference if their power level is too high.
An object of the present invention is to address the above problem.
According to a first aspect of the present invention there is provided a radio communication system comprising a primary station and a plurality of secondary stations, the system having a communication channel between the primary station and a secondary station, the channel comprising an uplink and a downlink control channel for transmission of control information, and a data channel for the transmission of data, wherein power control means are provided for adjusting the power of the control and data channels in a series of steps of variable size, the size of the power control steps being large initially and reducing as the power approaches its target value.
According to a second aspect of the present invention there is provided a primary station for use in a radio communication system having a communication channel between the primary station and a secondary station, the channel comprising an uplink and a downlink control channel for transmission of control information, and a data channel for the transmission of data, wherein power control means are provided for adjusting the power of the control and data channels in a series of steps of variable size, the size of the power control steps being large initially and reducing as the power approaches its target value.
According to a third aspect of the present invention there is provided a secondary station for use in a radio communication system having a communication channel between the secondary station and a primary station, the channel comprising an uplink and a downlink control channel for transmission of control information, and a data channel for the transmission of data, wherein power control means are provided for adjusting the power of the control and data channels in a series of steps of variable size, the size of the power control steps being large initially and reducing as the power approaches its target value.
According to a fourth aspect of the present invention there is provided a method of operating a radio communication system comprising a primary station and a plurality of secondary stations, the system having a communication channel between the primary station and a secondary station, the channel comprising an uplink and a downlink control channel for transmission of control information, and a data channel for the transmission of data, and at least one of the primary and secondary stations having power control means for adjusting the power of the control and data channels in a series of steps of variable size, the method comprising controlling the size of the power control steps to be large initially and reduce as the power approaches its target value.
In one embodiment of the present invention the power control step size is reduced when the sign of a power control command reverses, while in an alternative embodiment the power control step size is reduced after a predetermined time. These embodiments may be combined to reduce the power control step size when the first of the conditions is satisfied.
The use of more than one power control step size is known, for example from JP-A-10224294. However its use in this citation is limited to situations where power control is already established but propagation conditions are fluctuating rapidly. This citation does not address the problem of obtaining rapid convergence of power control at the start of, or after an interruption in, a transmission.